Schematics of POMC synthesis. Step 1: Synthesis of pre-POMC by reacting citric acid, maleic acid and 1,8-octanediol. Ratios of maleic acid and citric acid were varied as 10:0, 8:2, 6:4, 4:6 and 2:8, respectively, whereas the ratio of overall acid to diol was kept as 1:1. All the monomers were melted at 160°C under a nitrogen blanket. Further polymerization was continued at 135°C for 125 min to obtain a low-molecular-weight pre-polymer. Step 2: Photo-cross-linking of pre-POMC was performed under 365 nm UV light for 3 min in the presence of a photoinitiator (Igacure 2959).